Kangaroos use their tail as an extra leg when they walk, according to new research.
The study, reported in Royal Society Biology Letters, found that the animals use their tail more than their forelimbs when they walk.
The findings provide new insights into kangaroo locomotion, and could also have applications in advanced robotics.
Professor Terry Dawson of the University of New South Wales and colleagues have previously shown that during hopping, the kangaroo tail acts as a counterbalance, and as a spring to store up energy for the next bounce.
"Hopping is an exaggerated gallop, enabling the kangaroo to make longer steps," says Dawson.
In this study the researchers were interested in working out how kangaroos walk.
When kangaroos walk they had been observed to use what is known as a "pentapedal tail walk", says Dawson.
This unusual gait means the animal effectively always has three points on the ground, with one of them always being their tail, and the other two being either their two hind legs, or their two forelimbs.
However the energetics of pentapedal walking indicated that it was less energy efficient than walking on four legs, says Dawson.
"This was always a puzzle," he says.
While previous literature suggested kangaroos used their tail as a "strut to hold the body in place while they move the back legs forward," Dawson suspected something else was happening.
"It appeared to me they were using the tail for propulsion when walking," he says.
To test this hypothesis, Dawson and colleagues trained one male and four female red kangaroos (Macropus rufus) to walk over a force-measuring platform that monitored the energy used by different parts of the kangaroo as it walked.
The researchers found that kangaroos walk by using the tail to lift both hind legs and the body's centre of gravity forward, while the forelimbs were used as struts and didn't provide any of the propulsion.
The tests showed there was far more propulsion energy provided by the tail than scientists had thought.
The kangaroo's tail provides as much propulsive energy as one of the hind legs, between a quarter and a third of the full propulsion needed to move the animal forward, they found.
"We expected this is because the muscles in the tail and hind legs are highly aerobic, with a lot of mitochondria in them doing a lot of work," says Dawson.
Mitochondria act as a cell's power house, providing energy.
"The muscle structure of the front legs have little mitochondria and they're not organised for propulsion, so instead of the tail being the strut, the front legs were filling that role," says Dawson.
"I can now understand where that energy goes and why if they're going to walk more than 5 metres they get up and hop instead."
Dawson says the team is interested in mechanical analogues to walking and locomotion.
"There's interest in the robotic side of things and how other forms of locomotion can work," says Dawson.
"You can locomote just using your legs, but there are other options for stability and it's interesting from that point of view."